1. Field of the Invention
The present invention relates to power controllers, and more particularly to digitally controlled hysteretic dual mode power controllers, such as those used to power Subscriber Line Interface Circuits.
2. Related Art
Typical switch mode controllers use fixed frequency pulse width modulated (PWM) techniques for low power applications, such as flyback converters. These converters take an error signal generated from comparing a scaled output voltage to a reference voltage and amplify and filter the error signal.
This analog error signal is then compared to a current ramp for the primary inductor (in current mode controlled topologies) to modulate the primary inductor pulse width. This allows the output voltage error to program the primary inductor current and thus delivered power. This technique has shortcomings, such as control loop gain and bandwidth as a function of loading. At light loads, where these converters begin pulse skipping, stability problems arise. Compensating these converters for a wide dynamic range of power output becomes difficult. In addition, firing the primary switch every cycle at high frequencies dissipates excessive power relative to the light load condition, thus making the converter very inefficient at light loads. Therefore, light load converter stability and efficiency are two significant shortcomings of this approach.
An example of an application of such analog power controllers is in telephony, such as power controllers used to power subscriber line interface circuits (SLICs). The particular efficiency problem arises due to the difficulty of achieving good efficiency at all modes of operation over a wide range of load conditions. Specifically, while the power controllers (power converters) are relatively efficient in the off-hook state, they are typically very inefficient in the idle (on-hook) state. In typical household configurations, at nearly zero efficiency, under typical on-hook operating conditions, the power supply is wasting approximately 150-200 milliwatts.
Further, the SLIC needs to be able to operate during a lost power condition, i.e. it needs to operate from a battery power source. Under these conditions, the power controller's efficiency in the idle state becomes particularly important.
Accordingly, a need exists for a power output controller that is efficient at various load conditions, such as the on-hook and off-hook conditions in a telephone system, and has well-defined frequencies of operation.